Bottom Line:
An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release.The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages.This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

ABSTRACTTemporal fenestration has long been considered a key character to understand relationships amongst reptiles. In particular, the absence of the lower temporal bar (LTB) is considered one of the defining features of squamates (lizards and snakes). In a re-assessment of the borioteiioid lizard Polyglyphanodon sternbergi (Cretaceous, North America), we detected a heretofore unrecognized ontogenetic series, sexual dimorphism (a rare instance for Mesozoic reptiles), and a complete LTB, a feature only recently recognized for another borioteiioid, Tianyusaurus zhengi (Cretaceous, China). A new phylogenetic analysis (with updates on a quarter of the scorings for P. sternbergi) indicates not only that the LTB was reacquired in squamates, but it happened independently at least twice. An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release. The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages. This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

Mentions:
Despite the similarities noted above, the structure of the LTB in P. sternbergi differs from the one in T. zhengi, as in the latter the bar is firmly sutured to the quadrate, whereas in P. sternbergi there is no discernable articulatory facet on the quadrate. Additionally, on the ventrolateral face of the quadrate tympanic crest there is a rugose surface similar to the one on the cephalic condyle, at the level of the posteroventral process of the jugal (Supplementary Fig. S1). This suggests there was a soft tissue connection between the jugal and the quadrate, likely formed by a reduced quadratojugal ligament, which also connects the jugal to the quadrate in some extant lizard species, such as Corucia zebrata28. In a large number of lizards and snakes, however, this temporal ligament does not contact the quadrate, but rather the mandible2829, and it is termed a jugomandibular ligament; in such cases there are no rugose surfaces on the quadrate ventrolateral margin. We exclude the possibility of this being an attachment site for the Musculus adductor mandibulae externus superficialis (MAMES—Fig. 2) because of the similarity in texture between this surface and the one on the cephalic condyle, as well as the absence of such rugosity on the tympanic crest of observed specimens of Tupinambis teguixin, a taxon in which the MAMES is extremely well developed.

Mentions:
Despite the similarities noted above, the structure of the LTB in P. sternbergi differs from the one in T. zhengi, as in the latter the bar is firmly sutured to the quadrate, whereas in P. sternbergi there is no discernable articulatory facet on the quadrate. Additionally, on the ventrolateral face of the quadrate tympanic crest there is a rugose surface similar to the one on the cephalic condyle, at the level of the posteroventral process of the jugal (Supplementary Fig. S1). This suggests there was a soft tissue connection between the jugal and the quadrate, likely formed by a reduced quadratojugal ligament, which also connects the jugal to the quadrate in some extant lizard species, such as Corucia zebrata28. In a large number of lizards and snakes, however, this temporal ligament does not contact the quadrate, but rather the mandible2829, and it is termed a jugomandibular ligament; in such cases there are no rugose surfaces on the quadrate ventrolateral margin. We exclude the possibility of this being an attachment site for the Musculus adductor mandibulae externus superficialis (MAMES—Fig. 2) because of the similarity in texture between this surface and the one on the cephalic condyle, as well as the absence of such rugosity on the tympanic crest of observed specimens of Tupinambis teguixin, a taxon in which the MAMES is extremely well developed.

Bottom Line:
An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release.The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages.This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

ABSTRACTTemporal fenestration has long been considered a key character to understand relationships amongst reptiles. In particular, the absence of the lower temporal bar (LTB) is considered one of the defining features of squamates (lizards and snakes). In a re-assessment of the borioteiioid lizard Polyglyphanodon sternbergi (Cretaceous, North America), we detected a heretofore unrecognized ontogenetic series, sexual dimorphism (a rare instance for Mesozoic reptiles), and a complete LTB, a feature only recently recognized for another borioteiioid, Tianyusaurus zhengi (Cretaceous, China). A new phylogenetic analysis (with updates on a quarter of the scorings for P. sternbergi) indicates not only that the LTB was reacquired in squamates, but it happened independently at least twice. An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release. The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages. This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.